Processes for producing grain oriented silicon sheet steels, as used in transformers and other electromagnetic devices, must be carefully controlled to assure a maximum degree of an easily magnetized preferred orientation in the final product and accordingly optimum isotropic magnetic properties. Basically, current commercial processes involve production of a steel having a very carefully controlled chemistry, typically 2.5 to 3.5% silicon, 0.07 lto 0.12% manganese, about 0.025% sulfur, less than 0.03% carbon, and less than 0.015% phosphorus. Slabs of this steel are hot rolled to a hot-band gauge of about 0.08-inch. Thereafter, the hot-rolled steel is cleaned and cold rolled to final gauge using one of various practices. Usually, cold rolling involves two cold reductions to a final gauge of between 0.010 to 0.015 inch with an intermediate anneal. The cold rolled sheet is then given a decarburizing anneal to effect primary recrystallization and decarburization, and thereafter given a final, carefully controlled, high temperature box anneal to effect a secondary recrystallization to the preferred orientation.
The production of a well developed oriented microstructure is dependent not only on a careful control of the interrelated processing parameters, but also on a careful control of metal chemistry. For example, it is well known in the art that a fine, dispersed, precipitated phase, such as manganese sulfide, must be present during the final orientation anneal to function as a grain growth inhibitor, and accordingly promote an oriented secondary recrystallization. Without such a precipitate, the final anneal will cause considerable grain growth of the larger primary grains with little or no secondary recrystallization to an oriented microstructure. On the other hand, such a precipitated phase is not beneficial unless it is precipitated during or after hot rolling of the slab to sheet, because prior hot treatments, blooming, etc., will cause such precipitates to be rather large, and not well dispersed, throughout grains of the hot-rolled sheet. To avoid this problem, commercial practices utilize a relatively high temperature preheat for slab hot rolling, i.e., temperatures of about 2500.degree. F, in order to dissolve the particles of the precipitated phase to be thereafter re-precipitated during or after hot rolling.
Although the above high temperature hot rolling practice is effective in dissolving sulfides included in the steel, so that they might subsequently be precipitated, as grain growth inhibitors, this practice is costly, time consuming, and troublesome, particularly with respect to the destructive effects of such high temperatures on the slab reheating furnace ceramic lining and supporting structure, and to the excessive oxidation of the surfaces of slabs being so heated. Indeed, considerable research efforts have been directed towards development of processes not necessitating such high slab reheat temperatures. Several such processes have been proposed or patented. These processes usually rely on using other precipitates for grain growth inhibitors or combinations thereof. None of these processes however have been reduced to commercial practice.
Recently it has been taught (U.S. Pat. No. 3,671,337, Ko Kumai et al.) that lower slab reheating temperatures can be used by reducing the amount of manganese sulfide in the steel provided that an aluminum nitride phase is present in sufficient amounts to augment the manganese sulfide as a grain growth inhibitor. Although exceptionally good results can be achieved with this process, it is more costly due to the necessity for exacting controls and the fact that yield of prime product is somewhat low. In addition, the high aluminum content appears to be troublesome during teeming due to reoxidation thereof in the absence of extraordinary precautions during teeming.
In other more recent developments it has been taught (U.S. Pat. No. 3,802,937, D. M. Kohler et al) that lower manganese sulfide contents will provide sufficient grain growth inhibition thus permitting lower slab-reheat temperatures, provided that the steel's oxygen content is no greater than 0.0045%. It is believed that in the absence of oxide nucleation sites, the sulfide phase is more finely dispersed. To this end therefore, the steel must be vacuum treated after tapping to assure a sufficiently low oxygen content, a costly and time consuming process.